1. Field of the Invention
This invention relates to a receiver apparatus for a communications bus, and in particular to a management unit for a receiver apparatus on a communications bus.
2. Description of the Related Art
Local networks often make use of a communications bus over which a set of nodes communicates. A driver module in a controller node transmits step-change signals over the bus to receivers in remote controlled nodes. The step-change signal activates the multiplexed remote nodes connected to the bus, and the bus also selectively transmits signals from the remote nodes back to a receiver in the controller node.
Historically, in automotive applications, functions such as door locks, seat positions, electric mirrors, and window operations have been controlled directly by electrical direct current delivered by wires and switches. Such functions may today be controlled by ECUs (Electronic Control Units) together with sensors and actuators in a multiplexed Controller Area Network (CAN). The Controller Area Network (CAN) standard (ISO 11898) allows data to be transmitted by switching a signal, at a frequency of 10 Kbauds to 1 Mbaud for example, to the multiplexed receiver modules over the differential pair cable. The receiver modules may be actuators that perform a function, for example by generating mechanical power required, or sensors that respond to activation by making measurements and transmitting the results back to the ECU over the bus.
A variant on the CAN standard is the LIN (Local Interconnect Network) sub-bus standard (see ISO 7498), to provide connection to local network clusters. A LIN sub-bus system uses a single-wire implementation (enhanced ISO9141), which can significantly reduce manufacturing and component costs. Component costs are further reduced by self-synchronization, without crystal or ceramics resonator, in the controlled node. The system is based on common universal asynchronous receiver and transmitter serial communications interface (UART/SCI) hardware that is shared by most micro-controllers, for a more flexible, lower-cost silicon implementation. Other standards for step-change signals over a communication bus are the Flexray and MOST standards.
Typically, the controller node comprises a microcontroller unit (MCU) that generates the signals to be transmitted and processes the signals received. The controller node also includes a unit for selectively supplying power to the controlled nodes and other modules of the controller node and sending signals controlling the operating state of the controlled nodes and the other controller node modules. Typically, the complete vehicle system comprises more than one controller node and sub-networks.
Reducing power consumption of the nodes of such networks, especially of the controller node, is often critical, especially during waiting periods when the controlled nodes are inactive. This is the case in automotive applications, for example, when a vehicle is parked. The nodes are designed with various degrees of standby, sleep, and stop modes, in which part or all of the operating functions are halted or the power supplies to part of the modules within the nodes are switched off. However, waking the functions up and restoring supplies to the switched off modules to retrieve the normal run condition of the module or node introduces a greater or lesser delay that may be more or less acceptable for a given function.
Another driver for utilization of sleep and stop modes is the increasing demand on processing power for the CAN nodes, which has the effect of pushing the MCUs into faster and greater power-demanding processes. As more processing intensive MCUs are being utilized in CAN networks, deeper sleep states are required to avoid the excessive standby currents generated by such high powered controllers. As a result, system power controllers now allow very low sleep state currents in which power consumption is reduced and one or more clocks are stopped. Unfortunately, this trend increases the wake-up time of such powerful processors because the clocks need to restart and stabilize.
As will be appreciated, use of sleep and stop modes to decrease power consumption has created a problems in typical multiplexed bus systems with minimum controller wake-up requirements, and particularly in CAN networks where these minimum wake-up and response time are becoming increasingly shortened. This problem has been solved in the past by requiring that either the module be kept in a fully awake mode or that the first message off the bus be discarded while the controller recovers from its long wake-up period and then is resent after the module's MCU is fully awake. As a result of the deep sleep or stop modes of CAN nodes, the corresponding information in the first CAN message received off the bus is typically lost because that node's MCU cannot wake up sufficiently fast to decipher the first message. These solutions present significant reductions in performance and response time of the node.
In other solutions, an auxiliary MCU is used in conjunction with the main MCU within the node. Upon receiving the message off the CAN bus, the multiplex physical layer sends a “wake” message to the auxiliary MCU, which reacts more quickly to the wake message than the main MCU. The auxiliary MCU can remain in a sleep state, while retaining a relatively low quiescent current. The more powerful main MCU is typically shut down in this mode. Upon awakening, the auxiliary MCU enables the regulator to power up the main MCU. Thereafter, messages can be transmitted and received onto the CAN bus from the main MCU through the multiplex physical layer. However, adding an additional MCU increases costs significantly.
As can be seen, there is a need for CAN nodes with high performance MCUs that draw very low quiescent current in a sleep state, and yet can respond quickly to receive and decipher the first incoming message on the CAN bus. Prior solutions have required keeping the controller powered at all times, adding a second smaller controller for use while the more powerful controller awakens, or resending of the first message once the node's controller has awoken. As can be seen, there is a need for a more efficient solution that does not use the first message solely as a controller wake-up message.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced.